Unable to resolve name, VPN 3000

I am having an issue with an error message on a VPN 3000 which is in logs every 5 minutes, error message below:

28768 05/22/2009 11:06:11.580 SEV=4 DNS/6 RPT=20253

Unable to resolve hostname: <mailservername>

28768 05/22/2009 11:11:11.580 SEV=4 DNS/6 RPT=20253

Unable to resolve hostname: <mailservername>

I hoping somebody could help on this as I am unable to see why this is. The DNS server is correct in the system setting and I am able to ping the IP address, I am also able to ping the mail server by name/ip address.